1. Field of the Invention
The present invention relates to a frequency converter circuit for converting a modulated received signal to an intermediate frequency signal, and more particularly relates to a frequency converter circuit suitable for use in a mobile terminal in, for example, a mobile telecommunication system.
In general, in car telephone systems and other mobile telecommunication systems, a reference oscillation circuit is provided and a local oscillation signal for frequency conversion or a carrier signal for modulation is obtained based on an oscillation signal of this reference oscillation circuit.
In such mobile telecommunication systems, along with the increase in demand in recent years, a demand has arisen for more effective use of the frequency spectrum by reducing the interval between channels and by interleaving of frequency allocations.
In order to meet these demands, it is necessary to raise the frequency stability of the radio frequency signal (hereinafter referred to as the "RF signal").
In general, when sending an RF signal from a base station to a mobile terminal, it is possible to secure a high frequency stability. This is because it is possible to provide the base station with an oscillation circuit with a high precision of better than 0.1 ppm as the reference oscillation circuit.
As opposed to this, when sending an RF signal from the mobile terminal to the base station, a high frequency stability cannot be secured. This is because the ambient conditions of use of the mobile terminal are stringent, so the precision of the reference oscillation circuit is limited to about 3 ppm.
Therefore, to meet the above demands, it is necessary to enhance the frequency stability of the RF signal transmitted by the mobile terminal.
In conventional mobile terminals, the oscillation frequency of the reference oscillation circuit was designed to be controlled based on the RF signal sent from the base station.
If using such a construction, since the frequency stability of the RF signal sent from the base station is high, it is possible to also raise the frequency stability of the RF signal sent from the mobile terminal. Note that such a construction is adopted as is in the present invention as well (mentioned later).
In the above-mentioned mobile telecommunication systems, however, conventionally, the main signal, that is, the speech signal, has been treated through analog processing.
In such analog mobile telecommunication systems, use is made of the frequency modulation (FM) method as the modulation method, so not that high a frequency stability has been required of the received intermediate frequency (IF) signal.
2. Description of the Related Art
As opposed to this, in recent years, digital processing of a speech signal has been considered for such mobile telecommunication systems. In such digital mobile telecommunication systems, one type of phase shift keying (PSK) modulation method, i.e., the .pi./4 shift quadrature PSK (QPSK) modulation method, is in general use as the modulation method, and the received signal has been processed by a double superheterodyne receiving system. When use is made of the .pi./4 shift QPSK modulation method as the modulation method, the deviation of the frequency of a second intermediate frequency signal, the input signal of the demodulation circuit (hereinafter referred to as the "second intermediate frequency"), is converted to phase error. This increases tremendously the possibility of deterioration of the error rate of the demodulation output. Note that in a double superheterodyne receiving system, a first intermediate frequency signal exists before the second intermediate frequency signal.
With a frequency converter circuit in a conventional double superheterodyne receiving system, as will be explained later with reference to the drawings, even if it is attempted to stabilize the frequency of the first intermediate frequency signal, due to the features of the circuit construction, there is the problem that it is not easy to also satisfactorily stabilize the frequency of the second intermediate frequency signal.